Bars in Combat

AP - Usually blue. Used to execute Magic and regular attack powers. This goes down as you use your powers.

Shield - Usually purple. If you have a shield up any damage you take goes towards your Shield and not your HP then when the Shield is down the damage returns to your HP.

Group - Usually an off purple. You earn points while in a group that allow you to execute special powers.

Rage - Usually lime green. This bar fills up during a fight and auto triggers special damage bonuses.

Other bars you may see in combat include:

Big Red Turn bar at the top of the screen. You have a fixed amount of time to take your turn. Triggering an action completes your turn or not doing anything causes the game to let the monster take its turn when the big red timer bar at the top of the fight screen gets to 0.